Not Too Hot, Not Too Cold

In NREL's Advanced Power Electronics Lab, liquid cooling equipment is used to evaluate heat transfer in vehicle components.
Photo by Dennis Schroeder, NREL

Mythological character Icarus' melted wings sent him plummeting to earth when he ignored his father's advice and flew too close to the sun. Heat was Icarus' undoing. Today's real-world electric-drive vehicles (EDVs) also require diligent attention to temperature. The battery, power electronic system, electric motor operating temperatures, and climate control all factor into an EDV's performance, range, lifespan, affordability, and—most importantly—driver acceptance.

Last year, U.S consumers drove more than 487,000 EDVs (hybrid, plug-in hybrid, and all-electric vehicles) off dealers' lots. But most of these vehicles still can't match the price, driving range, and refueling speed Americans have come to expect from gas-powered automobiles. Issues with thermal management cause some of these limitations.

At the same time, as the U.S. auto industry grapples to meet ambitious new government fuel economy regulations, the U.S. Department of Energy (DOE) and the cross-agency EV Everywhere Grand Challenge initiative have set goals for EDVs that more than double driving range, cut battery and electric drive system costs by 75%, and use less energy to achieve the same level of climate control.

Experts at the National Renewable Energy Laboratory (NREL) are working closely with industry partners to address these thermal management challenges, spark greater consumer interest in EDVs, and put more fuel-efficient trucks on the road. Their research focuses on dramatically increasing energy efficiency, improving reliability, and decreasing emissions and cost, while maximizing vehicles' appeal to consumers.

"We can only meet new fuel-economy standards of 54.5 mpg by 2025 if we use a wide range of strategies, including broader deployment of electric vehicles," says Chris Gearhart, director of NREL's Transportation and Hydrogen Systems Center. "And we'll only be able to get drivers in those cars if we solve the temperature puzzle."

Batteries: Longer Range at a Lower Cost

Often the most expensive of EDV components, batteries need to be affordable, high-performing, and long-lasting to make these vehicles attractive to more consumers. According to EV Everywhere, if EDVs are to gain market share, batteries will have to cost less by a factor of 4 but take drivers twice the distance on a single charge. Understanding thermal characteristics is crucial to meeting these goals.

NREL, as a recognized leader in battery thermal management research and development (R&D), evaluates battery cells, modules, and packs. The lab's thermal behavior, capacity, conductivity, lifespan, and overall performance assessments factor in the impacts of full-system integration.

"The industry, with support of NREL and DOE, has made incredible progress—10 years ago these batteries were almost triple the cost and three times the size, but could only move a car half the distance," says NREL Energy Storage (ES) Group Manager Ahmad Pesaran. "That said, we still have a long way to go."

The laboratory's tests for the U.S. Advanced Battery Consortium show that optimized thermal management can increase battery power by more than 20%. Without proper thermal management, an EDV battery that can last almost 15 years in a temperate climate, like in Minnesota, lasts only seven years in a hot climate, such as in Arizona. In extreme instances, battery overheating can lead to issues such as those that have plagued the Boeing 787 Dreamliner, resulting in fire and, in rare cases, explosion of the battery material.

NREL's breakthrough research is focused on reducing thermal resistance of components to achieve more uniform temperatures. NREL uses its R&D 100 Award-winning Isothermal Battery Calorimeters, the only instruments in the world capable of such precise thermal measurements, for much of this research. NREL is also working with industry to develop computer-aided engineering software tools to optimize thermal management of batteries.

Power Electronics and Motors: Reduced Size, Weight, and Cost

Power electronics, which run a wide range of systems in conventional automobiles, are essential to EDV performance. Unfortunately, technology has yet to meet the demands of a mass-market audience.

Dramatic advances in power electronics and electric motors (PEEM) will be required to meet the EV Everywhere initiative's affordability and performance targets. Boosting electric-drive system efficiency, while reducing cost by 75%, and size and weight by more than 35%, will rely heavily on improved thermal management.

In EDVs, power electronics control the flow of electricity between the battery, the motor, and other powertrain components. The PEEM team improves thermal performance of components and systems through modeling, testing, and analysis. This leads to cooling systems and packaging materials that meet energy efficiency, performance, and reliability targets.

"Some of this technology has already been applied to commercially available components," says Advanced Power Electronics and Electric Motors Task Lead Sreekant Narumanchi. "We continue to work with partners in helping make PEEM components lighter, smaller, and less expensive, eventually helping make EDVs more competitive in the marketplace."

Climate Control: Improved Range and Thermal Comfort

Climate control systems such as air conditioners and heaters make both conventional vehicles and EDVs more comfortable. At the same time, electrical energy consumed for climate control can significantly reduce EDV range—in some cases by as much as 68%.

Conventional vehicles heat cabins with engine waste heat, but EDVs do not have an engine, which presents climate control challenges for automobile manufacturers. Using the battery for cabin heating takes valuable energy away from propulsion.

By improving thermal management, NREL researchers believe they can increase EDV range by 10% during operation of the climate control system. In collaboration with the automotive industry, the lab is exploring thermal load reduction technologies and improving efficiency while maintaining the thermal comfort that drivers expect. Strategies include:

Zone-based cabin temperature controls

Advanced heating and air conditioning controls

Seat-based climate control

Thermal load reduction

Thermal preconditioning.

"The impact of climate control on an electric vehicle can be significant depending on the temperature and driving conditions," says John Rugh, task leader for Vehicle Thermal Management. "Our work with industry partners aims to minimize energy for climate control so the battery can be used to power the wheels."

Integrated Thermal Management: Closing the Loop

NREL's Vehicle Testing and Integration Facility test pad features an on-site weather station to provide accurate data on local meteorological conditions.
Photo by Dennis Schroeder, NREL

By working to reduce the cost and increase the efficiency of EDV cooling systems, NREL is helping the automotive industry move closer toward the goals of extending battery life and driving range between charges, while improving safety, reliability, and comfort.

Vehicles with internal combustion engines use radiators and oil coolers to remove heat from the engine and transmission. EDVs, however, require more complicated systems to meet the additional thermal demands of power electronics and energy storage systems.

Using thermal testing and analysis, NREL is evaluating the potential benefits of combining the PEEM and ES cooling loops with the engine cooling and passenger compartment climate control systems. Reducing the number of cooling systems and related components can translate into lower component and maintenance costs, less weight, reduced aerodynamic drag, and ultimately better fuel economy and range.

NREL's thermal model of a compact-sized EDV has reduced total vehicle thermal management power consumption by combining cooling loops and using waste heat from PEEM components. Combined cooling loops use refrigerant-to-liquid heat exchangers, creating a more efficient system with improved heat transfer, as well as providing liquid to cool PEEM and ES systems.

Light-duty EDVs are not the only vehicles that can benefit from improved thermal management. According to an Argonne National Laboratory report, each year in the United States, long-haul trucks consume approximately 838 million gallons of diesel fuel for rest-period idling, much of which is used for heating and air conditioning. DOE's SuperTruck program has set a goal to improve heavy-duty vehicle fuel economy 50% by 2015, and addressing thermal management and climate control loads will be essential in achieving this. Working closely with industry partners, NREL's CoolCab program has shown that improved cab thermal management can reduce climate control loads and associated costs. This could cut fuel consumption, emissions, and operating costs.

"If we can demonstrate a three-year or better payback period with relatively low risk on these technology investments, truck operators will be economically motivated to adopt the technologies," says Jason Lustbader, CoolCab task leader. "Our goal is to bring down climate control loads by at least 30%."

Using truck cabs located on the Vehicle Testing and Integration Facility (VTIF) test pad, researchers investigate a wide variety of cabin thermal management technologies. Engineers quantify the impacts of different materials and equipment—films, paints, radiant barriers, and idle reduction technologies—on climate control loads.

As anyone who has been in a car on a sunny day can attest, dark paint colors absorb heat. The large painted surfaces of heavy-duty trucks further increase this effect. NREL researchers measured a 20% decrease in daily electrical air-conditioning system energy consumption after switching a truck's color from black to white. Engineers are also investigating advanced paints that look like darker colors, but thermally behave like lighter colors, giving truck fleets greater flexibility in selecting paint colors without sacrificing efficiency. Insulation is a factor as well. NREL tests have shown a 34% reduction in truck sleeper climate control loads using advanced methods of insulation.

Researchers and outside partners use NREL's CoolCalc and CoolSim modeling tools to simulate energy used for climate control in truck cabs and calculate the potential benefits of thermal load reduction options in a range of use and weather scenarios. The tools make it possible to rapidly evaluate the impact of factors such as insulation thickness, material properties, and geometries on climate control loads over the wide range of weather conditions experienced in real-world operation and identify the most promising solutions.

Turning Widespread EDV Adoption from Myth into Reality

NREL researchers are working to turn widespread adoption of energy-efficient vehicles from a myth into reality. Improving thermal management will result in the enhanced performance and reduced costs needed to motivate more drivers and operators to adopt EDVs and energy-efficient trucks. And that is likely to lead to a happy ending for consumers, the economy, and the environment.

Power Electronics and Electric Motors

By identifying and developing cooling systems and packaging materials, NREL has developed:

A light-weight, low-cost heat exchanger that reduces thermal resistance and increases power density (the rate of energy release per unit of component volume or weight) by more than 50% compared to commercial, state-of-the-art heat exchangers

As shown here, the test chamber of the large-volume IBC is big enough to accommodate the sizable battery modules used in electric-drive vehicles.
Photo by Dennis Schroeder, NREL

Developed by NREL in collaboration with industry partner NETZSCH, the R&D 100 Award-winning Isothermal Battery Calorimeters (IBCs) are the only ones in the world capable of performing the precise thermal measurements—with 98% accuracy—needed to make safer, longer-lasting, and more cost-effective lithium-ion batteries. Such precise heat-generation measurements are crucial for optimizing battery thermal-management systems to extend battery life and improve safety at affordable cost.

Lithium-ion batteries are widely considered the leading energy-storage option for electric-drive vehicles, but extreme temperature swings can diminish their performance, lifespan, and safety. Diagnostics performed using the IBCs provide the foundation for developing new thermal-management techniques that ensure the batteries meet warranty specifications and provide sufficient power.

The IBCs' patent-pending innovations—complete thermal isolation, the ability to test large cells and batteries, and the features necessary for testing high-power batteries safely—provide battery integrators with a level of accuracy and functionality not available in other calorimeters.

Made possible thanks to support from the U.S. Department of Energy's Vehicle Technologies Office, NREL's 15-year history of developing isothermal battery calorimeters and generating thermal data has helped the U.S. battery industry develop systems with better thermal performance. NETZSCH is in the process of commercializing the IBC technology following a 72% year-over-year jump in sales of hybrid-electric and plug-in vehicles.

In addition to earning a 2013 R&D 100 Award, which ranks the instruments among the year's most significant innovations, the IBCs were recognized with a 2012 Governor's Award for High Impact Research.